Field of the Invention
The invention relates to a lens, and more particularly to a projection lens.
Description of Related Art
Along with the development of the displaying technology, projectors are widely applied in different occasions, such as the conference or the office meeting. Mostly, the projectors are used for image displaying or video playing. An important feature for the projector is that the image can be enlarged and presented on the screen. However, the distance between the projector and the screen is required to be small enough to prevent waste of space. Therefore, a projection lens employed in the projector has to be capable of providing a large image within a limited range.
To design a projection lens for wide angle display, the aberrations such as distortion, field curvature or astigmatism should be taken into account. Several methods are already proposed to lower the aberration effects such as extending the length of the projection lens, employing the combination of spherical lenses and aspheric lenses for the projection lens or increasing the number of lens. However, those methods also increase the difficulty of designing each element in the projection lens. Therefore, how to provide a projection lens which can be easily designed for wide angle display with low aberration effect is still a main topic for research and development.
In the known art, a variety of methods to improve the imaging quality of the wide-angle projection lens are disclosed. U.S. Pat. No. 7,009,765 uses a secondary imaging principle to produce an intermediate image, thereby reduce the aberration problem. U.S. Pat. Nos. 5,477,394 and 6,994,442 disclose a scheme that by using a plurality of reflectors to shorten the projection distance. U.S. Pat. Nos. 6,896,375, 7,341,353, 7,545,586 and 7,567,380 use multiple reflectors to shorten the focal length. U.S. Pat. No. 7,150,537 and U.S. Patent Application No. 20100172022A1 use positive-negative distortions complementary principle of two groups of lenses to correct the aberration. U.S. Pat. Nos. 4,427,274 and 5,495,306 propose a projection lens. U.S. Pat. No. 6,631,994 discloses a refracting optical lens to correct for pincushion distortion of the convex mirror. U.S. Patent Application No. 20100039625 discloses a projection optical system for use in an image projection apparatus. In addition, Taiwan Patent No. 436661 discloses a curved surface reflective-type projection structure of digital projector.